Composite photoconductive layer



Jan. 3A, 1961I s. v. FoRcauE 2,967,254

COMPOSITE PHoTocoNDUcTIvE LAYER Filed Feb. 18, 1955 /f in lll/l as" a500D 1//070- nited States Patent i COMPOSITE PHOTOCONDUCTIVE LAYERStanley Vincent Forgue, Cranbury, NJ., assignor to Radio Corporation ofAmerica, a corporation of Delaware Filed Feb. 18, 1955, Ser. No. 489,203

11 Claims. (Cl. 313-65) 'the visible portion of the color spectrumcoupled with ioptimum time lag characteristics and good sensitivity.

It is therefore a principal object of this invention to :provide a newand improved photoconductive target having high sensitivity in thevisible portion of the spectrum -coupled with improved lagcharacteristics.

It is another object of this invention to provide an improvedphotoconductive target having suicient re- .sponse in the red portion ofthe visible color spectrum coupled with optimum sensitivity.

It is a further object of this invention to provide a new and improvedtelevision pickup tube utilizing a target having usable lagcharacteristics coupled with high sensitivity.

These and other objects are accomplished in accordance with thisinvention by providing a pickup tube having, within an evacuatedenvelope, a target including a porous layer of photoconductive materialand a solid layer of photoconductive material. The target may alsoinclude still another layer of photoconductive material.

The novel features which are believed to be characteristic of thisinvention are set forth in the appended claims. The invention itselfwill best be understood by reference to the following specification whenread in connection with the accompanying single sheet of drawings, inwhich:

Figure 1 is a sectional view of a pickup tube utilizing aphotoconductive target in accordance with this invention;

Figure 2 is an enlarged fragmentary sectional view of thephotoconductive target schematically shown in Figure 1;

Figures 3 4 are enlarged fragmentary sectional views of embodiments ofthis invention; and

Figure 5 is a light characteristic of a target in accordance with thisinvention.

Although this invention relates to photoconductive targets for use invarious types of electron discharge devices, it will be explained withreference to television pickup, or camera, tubes in which it isparticularly useful.

Referring now to the drawings in detail, and particularly to Figure 1,pickup tube comprises a vacuum tight envelope 11 having an electron gun12 mounted in one end thereof. The several electrodes of the electrongun 12 include the usual cathode 36, control electrode 37, and one ormore accelerating anodes 38 which are connected through lead-in pins 39to appropriate sources of potential (not shown) for forming an electronbeam 25. The specic design of the gun 12 is conventional and forms nopart of the present invention and further discussion thereof is notdeemed necessary.

The electron beam 25 from the gun 12 is directed upon a target y13mounted in the other end portion of ,i 2,967,254 Patented Jan. 3, 1961ICC tube 10. Target 13 is schematically shown in Figure 1 and will bedescribed in detail in connection with Figure 2. Means are provided forfocusing the electron beam and scanning the beam over target 13 to forma raster. These means may include a focus coil 14, deflection yoke 15,and an alignment coil 16. A mesh screen electrode 17, which is permeableto the electron beam 25, is positioned adjacent to target 13 and duringoperation, together with focus coil 14, functions to insure that theelectron beam 25 is normal to the surface of target 13 during itsapproach to the target 13. On the interior surface of envelope 11 is anaccelerating electrode 19 which is in the form of a conductive coatingon the envelope 11.

The above recited elements of the tube 10 are conventional and form nopart of this invention, except insofar as these elements are combinedwith the new and improved target electrode of this invention.

Referring now to Figures 1 and 2 for a description of target 13. Target13 is conventionally supported adjacent to a transparent window 20 andis electrically energized by means of a lead-in 26 extending through theWindow 20, or through other portions of the envelope 11.V The lead-in 26is connected to an appropriate potential source (not shown). As shownmore clearly in Figure 2, target 13 comprises a support plate 21 whichis preferably of transparent glass.- The support plate 21 may comprisethe transparent window 20, shown in Figure l, or the support plate 21may be a separate transparent support member which is supported by thelead-in 26 adjacent to the window 20. A transparent conductive coating,or signal plate 22 is disposed on the surface of support plate 21 towardthe electron gun 12, and may be of a material such as tin chloride ortin oxide.

In accordance with this invention, the target 13 includes a porouslayerof photoconductive material 23 and a solid layer of photoconductivematerial 24. In the embodiment of the invention shown in Figure 2 theporous layer of photoconductive material 23 is deposited on the surfaceof the transparent conductive coating 22, while the solid layer ofphotoconductive material is deposited on the exposed surface of theporous layer of photoconductive material 23. Examples of materials forboth the solid and porous layers of photoconductive material 23 and 24are antimony tri-sulphide,

cadmium selenide, germanium sulphide, and antimony` thiosulphide. Boththe solid layer and the porous layer of photoconductive material may bedeposited by evaporation.

A specific method of manufacturing the target 13 is to apply thetransparent conductive coating 22 ontoy the supporting plate 21 byspraying the tin oxide and condensing it on the support plate. Afterthis is done the support plate 21 is placed in an atmosphere of argon ata pressure within the approximate range of 10.-1 to several millimetersof mercury at which time a layer of antimony tri-sulphide is depositedby evaporation from a heated source of the material. Because of gaseousatmosphere the antimony tri-sulphide is deposited in a porous form.Next, the support plate is placed in a good vacuum of approximately 510-6, or higher, millimeters of mercury, and a second layer of antimonytri-sulphide is deposited by evaporation.

This second layer is of a solid form due to the high vacuum used. Theantimony tri-sulphide may be evaporated by heating a crucble containingthe material to a temperature within the range of 500 to 600 C. Both ofthe layers may be evaporated from the same crucble (not shown).Approximately milligrams of antimony tri-sulphide in'thercrucible hasbeen found to be sufficient to deposit both of the photoconductivelayers'.

and form the composite photoconductive layer for a target of l squareinch area.

A porous layer of cadmium selenide, may be deposited by `evaporating thematerial in an atmosphere of argon at apress'ure from-2X 1,0-1 toseveral mm. of mercury, while a porous ,layer of germanium sulphide maybe deposited by evaporating the material in an atomsphere of argon at a.pressure of 5 1Ol mm. of mercury or lower, and a porouslayer of antimonythiosulphide may be deposited by `evaporating the material in anatmosphere of argon at a `pressure of approximately 5 l01 mm or lower.The solid layers of cadmium selenide, germanium sulphide, and antimonythiosulphide may be deposited by evaporation in a good vacuum of theorderof lil-6mm. of mercury.

VIt should be understood that other atmospheres may be utilized whileforming the porous layer of photoconductive material 23 examples ofwhich are neon, oxygen,

I' lau.

,Physically a porous antimony tri-sulphide layer, i.e. one evaporated-ina poor vacuum, may be said to have a dull or smokey appearance whichcontrasts with the smooth and shiny surface obtained when the samematerial is evaporated in a higher vacuum. This porous layer may also bedescribed as spongy, or soft, to distinguish it from the solid layerwhich has a hard surface.

The thicknesses of the layers of photoconductive materil are notcriticalfor purposes of this invention. It has been found that a solidlayer of antimony tri-sulphide within the approximate range of .0l to .4Vmil operates satisfactorily Vin the pickup tube target previouslydescribed. The porous layer of antimony tri-sulphide may also be withinthe vsame approximate range and operate satisfactorily in the type oftarget under consideration. The comparative thicknesses between the twolayers of photoconductive material are also not critical. However, toeliminate capacity lag when the target described is used in a televisionpickup tube, it is necessary that the thickness of the compositephotoconductive layer be greater than a minimum value. By capacity lagis meant the lag caused by a charge being built up across the targetbecause of the capacity thereof. Since the capacity is increased as thetarget is made thinner, a minimum total thickness of 0.1 mil ispreferred in order for the target to reproduce a transient change inlight, i.e. the ability of the target to erase a signal in a givenperiod of time without showing a shadow of trail or light. Thisthickness given is based on a targetarea of about a square inch. Greaterthicknesses of layers would be necessary with larger area targets.

A target in accordance with the invention has a better response in thered portion of the spectrum than a target comprising a porous layeralone. This feature is shown schematically in Figure which compares thecharacteristics of a solid, porous, and composite layer of antimonytri-sulphide targets. `Figure 5 also shows the comparative sensitivitiesof porous, solid, and composite layers of antimony tri-sulphide inmicroamperes per microwatt of radiant energy. As is well known in theart, the total sensitivity of each of the layers over the visiblespectrum is a function of the area under the respective curve of Fig. 5,the response of the human eye and the characteristie of a standardtungsten light source, and is measured in microamperes per lumen. Theporous target has a sensitivity of the order of 50 to 75 micro amperesper lumen. A solid target alone has a sensitivity of 300 to 400 microamperes per lumen, however, the lag characteristics thereof areundesirable. The composite target has a sensitivity of approximately 200micro amperes per lumen in combination with good lag characteristics.

Furthermore, composite layer targets in accordance with this invention,have good photoconductive lag characteristics, i.e. they are capable ofreproducing pictures of fast moving objects in approximately l frametime. The photoconductive lag characteristic` of thenovel composite .4lantimony tri-sulphide target such as is shown in Figure 2 is betterthan that of a solid antimony tri-sulphide target alone and is as goodas, or better than, the lag characteristics of a porous antimonytri-sulphide target alone. Still further, the exposed surface of thecomposite target is mechanically rugged, while the composite targetthickness is suitable for the avoidance of capacity lag.

Referring now Vto Figure 3V there isV shown anenlarged fragmentarysectional view of an embodiment of this invention.` This embodimentof atarget 2S differs from that previously described-in that a solid layerof photoconductive material 27 is deposited on the signal plate "22. Onthe solid layer of photoconductive material 27 is deposited a porouslayer of photoconductive material 29. The means for depositing thelayers 27 and 29 as well as the operation and advantages of thecomposite layer may be similar to those previously described,

Referring now to Figure 4 there is shown an enlarged fragmentarysectional View of another embodiment of.

this invention. In this embodiment, a target 30 is provided for a pickuptube as shown in Figure l, whichicornprises a support member 21 having asignal plate 22 on one surface thereof. On the signal plate 22 isdeposited a porous layer of photoconductive material 32. On the porouslayer of photoconductive material `32 is a solid layer ofphotoconductive material 33. On the exposed surface ,of the .solid layerof photoconductive material 33 is deposited a third layer ofphotoconductive material 35. The third layer of photoconductive material35 may be a porous layer of photoconductive material, or in thealternative may be material different from those described above such asselenium, or 4other photoconductive material.

Itshould be understood that it is lwithin the contemplation of thisinvention that the order of photoconductive layers in the embodimentshown in Figure 4 maybe changed. The means for depositing the porouslayer 32 yandsolid layer 33, as well as the operation andadvantages ofthe triple layer target may be similar to those pointed out above. Thethird layer of photoconductive material 35 may be deposited byevaporation.

In each of the embodiments shown in Figures ,3 Vand 4f thephotoconductive materials may b e those described in connection withFigure 2, i.e. antimony tri-sulphide, cadmium selenide, germaniumsulphide, or antimony thiosulphide. It should be understood that theporous layery of photoconductive material may be constructed of adifferent material than the solid layer in each of the em` bodimentsshowneg. in Figure 2 the porous photoconductive layer 23 may be antimonytri-sulphide while the solid photoconductive layer 24 may be leadselenide .or cadmium selenide.

What is claimed is:

`l. A photosensitive device comprising, a porous photoconductivematerial, a solid photoconductive material extending over said porousphotoconductive material, and a conductive electrode extending over oneof said photoconductive materials. g

2. A photosensitive device comprising, a porous layer of photoconductivematerial, a solid layer of photoconductive material extendingsubstantially over said porous layer of photoconductive material, atransparent signal electrode on one of said layers of photoconductivematerial, and a member supporting said layers.

3. A photosensitive device comprising, a porous layer of antimonytri-sulphide, a solid layer of antimony trisulphide deposited on saidporous layer, a signal electrode on one of said layers of antimonytri-sulphide, and a member supporting said layers.

4. A target for a photoconductive camera tube-comprising, a signalelectrode, a porous layer of photocon-v ductive material disposed on onesurface of said signal electrode, and a solidlayer ofphotoconductive-materi-al disposed on said porous layer.

5. A .target for ,a i photoconductiye lcarriera tube com.

prising, a signal electrode, a solid layer of photoconductive materialdisposed on one surface of said signal electrode, and a porous layer ofphotoconductive material disposed on said solid layer.

6. A target for a television pickup tube comprising, a porous layer ofphotoconductive material, a solid layer of photoconductive materialextending over said porous layer of photoconductive material, and athird layer of photoconductive material extending over one of saidlayers of photoconductive material, and a signal plate extending overthe other of said layers of photoconductive material.

7. A target for a pickup tube comprising a porous layer of antimonytri-sulphide, a solid layer of antimony tri-sulphide extendingsubstantially over said porous layer of antimony tri-sulphide, and asignal electrode extending substantially over one of said layers ofantimony trisulphide.

8. A target for a photoconductive camera tube comprising a signalelectrode, a porous layer of antimony trisulphide disposed substantiallyover one surface of said signal electrode, and a solid layer of antimonytri-sulphide disposed substantially over said porous layer of antimonytri-sulphide.

9. A target for a camera tube comprising a transpareut signal electrode,a solid layer of antimony tri-sulphide disposed over one surface of saidsignal electrode, a porous layer of antimony tri-sulphide disposed oversaid solid layer of antimony tri-sulphide.

10. A photosensitive device comprising, a porous photoconductivematerial, a solid photoconductive material in Contact with said porousphotoconductive material, and a conductive electrode in contact with oneof said photooonductive materials, said photoconductive materials beingselected from the group consisting of antimony tri-sulphide, cadmiumselenide, germanium sulphide and antimony thiosulphide.

1l. A photo sensitive pick-up tube having an envelope the end Wall ofwhich comprises a light transparent window, a light translucentconductive coating on the inner surface of said window, a lighttranslucent porous layer of a photo-conductive material on saidconductive coating and a solid layer of a photo-conductive material inContact with said porous layer and which exhibits the photo conductiveeffect due to light reaching it through 20 said porous layer.

Weimer Aug. 24, 1954 Forgue May 8, 1956

